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Toolbox oracle-db-lThese are parallel processes. This generally happens when parallel querries are running on the database. This is achieved by a various factors like init.ora parameter parallel degree setting for tables indexes etc.
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As mentioned above, these are processes related to parallel processing on your database. I'm just curious here, why do you say there are "too many" of these parallel processes? Is there any sort of performance impact on the server? Also, what sort of jobs/processes are running on your database when you observe this, any large batch processes, for example?

In all probability, something has been designed to run in parallel (since you say they disppear. Did you check these sessions/processes are trying to do? As noted above, you may not be worried unless it has an impact on the performance.